This is still less safe and less convenient than sandboxing. I want them to be able to run whatever command they want inside the sandbox even if it's traditionally dangerous (overwrites files, etc) because it can't touch anything that matters and I can revert any changes to the code myself.

If anything the only thing you'd have to have reviewed is sandbox escape attempts.

You are right. And the hook is for convenience, not for safety.

Having explicit sandboxes is good, of course. But in addition I also treat my whole computer like a sandbox, it least in the sense that there's no files I would mind if they were overwritten or deleted: you should have backups anyway.